I’ve posted twice before on Calvin’s views on prescribed hours of prayer, here and here. The general view that emerged there was that no time is a bad time for prayer–but that such helps as we can find as inducements to actually doing it are of great use. Just as fixed hours for public worship are of great use, so also are fixed times for private prayer.
Both of those previous posts were on passages from his commentary on the Psalms. It is well known that Calvin intended his commentaries to be read in tandem with his Institutes of the Christian Religion, because the former provided the exegetical basis of the latter. And in this case (as in many others), we find that the two complement each other nicely.
Thus, Calvin in Institutes 3.20.50:
But although it has been said above (sec. 7, 27, &c.), that we ought always to raise our minds upwards towards God, and pray without ceasing, yet such is our weakness, which requires to be supported, such our torpor, which requires to be stimulated, that it is requisite for us to appoint special hours for this exercise, hours which are not to pass away without prayer, and during which the whole affections of our minds are to be completely occupied; namely, when we rise in the morning, before we commence our daily work, when we sit down to food, when by the blessing of God we have taken it, and when we retire to rest.
Nevertheless, observing hours does not suddenly relieve us of our ever-present duty. Such a practice should rather spur us on to greater attention to that duty: it is a “discipline by which our weakness is exercised”:
This, however, must not be a superstitious observance of hours, by which, as it were, performing a task to God, we think we are discharged as to other hours; it should rather be considered as a discipline by which our weakness is exercised, and ever and anon stimulated. In particular, it must be our anxious care, whenever we are ourselves pressed, or see others pressed by any strait, instantly to have recourse to him not only with quickened pace, but with quickened minds; and again, we must not in any prosperity of ourselves or others omit to testify our recognition of his hand by praise and thanksgiving.
For though we may bind ourselves to certain times as a matter of spiritual training, God himself is not so bound. And so Calvin closes this section by emphasizing the radical freedom of God: our own rules and prescriptions do not circumscribe the action and activity of the Holy One, and we must be wary never to presume to legislate to the divine lawgiver or pretend to ourselves that we can hem him in by “method, time, and place.” God remains unhindered in his simultaneously dreadful and paternal sovereign majesty:
Lastly, we must in all our prayers carefully avoid wishing to confine God to certain circumstances, or prescribe to him the time, place, or mode of action. In like manner, we are taught by this prayer 1 not to fix any law or impose any condition upon him, but leave it entirely to him to adopt whatever course of procedure seems to him best, in respect of method, time, and place. For before we offer up any petition for ourselves, we ask that his will may be done, and by so doing place our will in subordination to his, just as if we had laid a curb upon it, that, instead of presuming to give law to God, it may regard him as the ruler and disposer of all its wishes.